2753
used value of 18.5, the discrepancies noted between A pK value of 11.14 for malononitrile in aqueous solution was determined by Bowden and S t e ~ a r t . ~ the scales in dimethyl sulfoxide and cyclohexylamine solutions no longer appear, and, in fact, the agreement, This acid was then used as the “anchor compound” as shown in Table I, is even better than might have to establish an acidity scale in ethanol-dimethyl sulfbeen expected for two such different solvents. oxide mixtures. The scale was applied to the measurement of pK values for a number of carbon acids, among Calvin D. Ritchie, Ronald E. Uschold which were 9-carbomethoxyfluorene, tris(p-nitropheny1)Department of Chemistry State Unioersity of New York at Buffalo methane, and 9-phenylfluorene. The pK(H20) values Buffalo, New York 14214 shown in the table for these compounds are those obReceived February 23, 1967 tained by Bowden and Stewart. The present data clearly show the breakdown of the acidity scale. The relative acidity of malononitrile and A Novel Method for the Synthesis of Isomerically 9-carbomethoxyfluorene changes by 2.5 pK units on Pure Vinyl Halides from Alkynes uia the going from ethanol to dimethyl sulfoxide solution. The Hydroalumination Reaction’ relative acidities of 9-carbomethoxyfluorene and either Sir: tris(pnitropheny1)methane or 9-phenylfluorene, however, are nearly the same in both determinations. The addition of bromine to vinylorganoboranes In now appears probable that the agreement preresults in the formation of 1,2-dibromoboranes. These viously reported2 between our “absolute” pK‘s and derivatives are converted to vinyl halides when treated those determined by Steiner6 by acidity function techwith water or aqueous sodium hydroxide.2 Recently, niques is purely fortuitous. Apparently, the reference Matteson and Liedtke3 have suggested that bromine compound, 4-nitroaniline, used by Steiner for determiadds trans to unsaturated boronic esters, and they have nation of the other acids just happens to be one for established that deboronobromination involves a stereowhich the pK is not strongly affected by the change of specific trans elimination. In concurrence with these solvent. Malononitrile appears to be another such results is the observation by Brown2 that bromination compound, showing the same pK in water and dimethyl deof trans-l-hex-l-enyl-bis(3-methyl-2-butyl)borane, sulfoxide. rived from hydroboration of l-hexyne with bis(3-methylAs we discussed in our previous paper,2 four distinct 2-b~tyl)borane,~ gives after hydrolysis cis-l-bromo-leffects are expected to influence the change in acidity hexene. of an acid on going from water to dimethyl sulfoxide. We have now investigated the halogenation of vinylTwo of these effects, the greater basicity of dimethyl alanes. These derivatives are readily available by sulfoxide than water and the dispersion interaction of hydroalumination of alkynes with diisobutylaluminum highly colored anions with solvent, would act to inhydride in a hydrocarbon so1vent.j This involves a crease acidity in dimethyl sulfoxide. The other two cis addition of the aluminum-hydrogen bond to the effects, electrostatic effects and hydrogen bonding of triple bond, yielding trans-vinylalanes from 1-alkynes solvent to the conjugate base, would decrease acidity and cis-vinylalanes from disubstituted alkynes. Treatin dimethyl sulfoxide. For the nitroanilines and for ment of the trans-vinylalane derived from I-hexyne in malononitrile it appears that these effects just balance. tetrahydrofuran with various halogens in a 1: 1 ratio We believe that this cancellation is purely fortuitous at - 50” produces essentially pure trans-l-halo-l-hexand even results from different magnitudes of the inenes. Under similar experimental conditions the cisdividual effects in the two cases. Both hydrogen bondvinylalane obtained from 3-hexyne reacts with bromine ing and the dispersion interactions are expected to be or iodine to give the corresponding cis-3-halo-3-hexgreater for the nitroanilines than for malononitrile. enes. The experimental results are summarized in The increase in acidity of 9-carbomethoxyfluorene Table I. over that for malononitrile in going from ethanol to The fact that halogenation of unsaturated alanes dimethyl sulfoxide is most reasonably attributed to the proceeds with retention of configuration supports the dispersion interactions of the substituted fluorenyl contention that vinylalanes undergo electrophilic cleavanion with the polarizable solvent. age (1) preferentially at the vinyl carbon-aiuminum Two conclusions pertinent to current investigations bond. Formation of vinyl halides from vinylboranes, in the study of acidities of hydrocarbons can be drawn however, is the result of an addition-elimination refrom our present data. First, and in our opinion most action (2). It should be noted that the reaction of important, the data clearly show that no single Hchlorine with the vinylalane derived from 1-hexyne acidity function applicable to acids of different structures gives a 70 :30 mixture of trans- and cis-l-chloro-lcan be constructed to reach into pure dimethyl sulfoxide. hexene, indicating a competition between addition to This conclusion is further strengthened by the results the double bond and cleavage at the vinyl carbonof Steiner’s recent study.’ aluminum bond. Second, if the relative acidities of various hydro(1) This research was supported by National Science Foundation carbons in cyclohexylamine solution, determined by Grants No. GP3521 and GP 6633. (2) B. M. Mikhailov and P. M. Aronovich, Izu. Akad. Nuuk S S S R , S t r e i t ~ i e s e r ,are ~ adjusted to our value of 16.4 for the Old. Khim. Nauk, 927 (1961); Chem. Abstr., 55, 24541 (1961); D. S. pK of 9-phenylfluorene, rather than to the previously (5) K. Bowden and R. Stewart, Tetrahedron, 21, 261 (1965). (6) E. C. Steiner and J. M. Gilbert, J . Am. Chem. Soc., 87, 382 (1965). (7) E. C. Steiner and J. D. Starkey, ibid., 89,2751 (1967). We wish to express our appreciation to Dr. Steiner for furnishing us with a preprint of this paper.
Matteson and K. Peacock, J . Org. Chem., 28,369 (1963); W. G. Woods and I. S . Bengelsdorf, ibid., 31,2766 (1966); H. C. Brown, privatecorn; munication. (3) D . S. Matteson and J. D. Liedtke, J . Am. Chem. Soc., 87, 1526 (1965). (4) H. C. Brown, and G. Zweifel, ibid., 83, 3834 (1961). (5) G. Wilke and H. Muller, Ann., 629, 222 (1960); G. Zweifel and R. B. Steele, to be published.
Communications to the Editor
2754 Table I. Halogenation of Vinylalanes in Tetrahydrofuran Vinylalane derived from 1-Hexyne
Halogena 12
IC1 Brz 3-Hexyne
I2
Brz
Vinyl halide, trans
cis
94 76d 72
